1. Field of the Invention
The invention relates to a method of suppressing noise in a digital signal obtained by sampling an analog signal, more specifically a television signal, as well as an arrangement for implementing the method.
When sampling an analog signal, that is to say, when taking samples at continuous time intervals, at the so-called sample clock, the analog signal should ideally be represented accurately by the digital sample values. With a constant analog signal, the successive sample values should then always present the same value. But in practice, this ideal case can hardly be attained because noise is superimposed on the analog signal when adjusted, or disturbing noise appears during the sampling operation. This entails that, for example, with a constant analog signal, the digital sample values repeatedly assume changing magnitudes.
In the case of television signals, when in a picture picture-areas often appear having the same contents, this kind of noise or its entailing errors appears very clearly in the sample values.
2. Summary of the Invention
It is an object of the invention to provide a method of and an arrangement for suppressing noise of a digital signal, which method and arrangement are specifically suitable for digital television signals and cause a noise reduction of the digital signals, especially for sampled constant analog signals.
This object is achieved when implementing the method of suppressing noise according to the invention in that for each two successive sample values, the second sample value is replaced by the preceding sample value if the difference between these two falls short of a predetermined difference of value.
Depending on the range of application, a difference of value may be predetermined at which the successive sample values are no longer changed when a digital signal recovered from an analog signal by means of sampling falls short of this difference. This means that always when considering two sample values, every second value is assumed to be equal to the first when the difference between the two sample values does not exceed the predetermined difference. Only when this value difference is exceeded is the original magnitude of the second sample value retained. In this manner an effective noise reduction is achieved.
The magnitude of the predetermined value difference can be selected depending on the purpose for which the method is utilized. When, for example, implementing the method of reducing noise of a television signal, the value can be selected such that, on the one hand, not too many details are lost and, on the other hand, a proper noise reduction is effected.
Furthermore, the method is advantageous in that signal jumps are taken over in an undistorted manner; so the pulse behavior of the signal is not changed.
According to a further embodiment of the invention, it is provided that if a difference is found between the sample values in a predeterminable tolerance range around the predeterminable value difference, the second sample value is replaced by the mean value of the second sample value itself and the preceding sample value.
In this respect, in addition to the decision whether the available difference between two sample values exceeds or falls short of the predetermined difference value, another tolerance range around the predetermined difference value is considered. If the available difference value is situated in this tolerance range, the mean value is computed from the second sample value and its preceding sample value in order to replace the second sample value. The advantage of this procedure is that if a difference value is situated in the predeterminable tolerance range, the sample values more and more approach the correct value. For example, it may occur that the first sample value of a constant analog signal happens to be distorted by noise and is consequently erroneous. If the next sample values do not yet exceed the predeterminable value difference, the faulty value will then be taken over again and again. If a tolerance range is additionally provided around the predeterminable difference value and the procedure takes place in the above manner, the sample values are brought to the right level.
In a further embodiment of the invention, an arrangement for implementing the method comprises a multiplexer, to whose first input the unchanged sample values are applied, a delay element following this multiplexer and delaying each sample value by one sample clock and whose output signal is applied to a second input of the multiplexer, a subtractor to which the unchanged sample values as well as the output signal of the delay element are applied, and whose output signal, subsequent to a quantity formation, is applied to a comparator that compares the applied values to a predeterminable difference value, on which the multiplexer is switched to its first input if the predeterminable difference value is exceeded, and is otherwise switched to its second input.
Thus, the difference value between each actual, unchanged sample value and a sample value delayed by one sample clock, i.e. the preceding sample value, is applied to the comparator. The comparator compares this difference value to a predetermined difference value and controls the multiplexer in a manner such that it switches the actual sample value to its output only if the predeterminable difference value is exceeded by the two sample values. The multiplexer is otherwise switched to its second input, to which the output signal of the delay element is applied, that is, the sample value of the preceding sample clock. Then the desired noise reduction is effected.
According to a further embodiment of the invention the multiplexer has a third input to which the mean value of the actual, unchanged sample value and the output signal of the delay element is applied, and in that the multiplexer is switched to the third input if a difference value is situated in the tolerance range.
The above-mentioned improved approximation to the actual signal value can be achieved by using the arrangement, in that the third input of the multiplexer is activated just when the difference between the successive sample values is exactly situated in the tolerance range. In that case, the mean value of each actual, unchanged sample value and the output signal of the delay element is switched to the output of the multiplexer. It has the above-described consequence that if an initial value happens to be erroneous, the subsequent values are gradually brought up to the right level. But at the same time the effect of noise suppression is retained.
According to a further embodiment of the invention, an arrangement for implementing the method comprises, a multiplexer, to whose first input the sample values multiplied by a factor ##EQU1## are applied and whose output signal is applied to its second input after a delay by one sample value and an addition by sample values delayed each time by one sample clock, and a subtractor to which the unchanged sample values and the sample values delayed by one sample clock are applied, and whose output signal, after quantity formation, is applied to a comparator that compares these to a predeterminable value difference value, upon which the multiplexer is switched to its first input if the difference value is exceeded, and is otherwise switched to its second input.
This arrangement operates in the same manner as the one described hereinbefore, insofar here too the actual sample value and a sample value delayed by one sample clock are compared and in response to this result the multiplexer switches either the actual or the delayed sample value to the output of the arrangement. But furthermore, a feedback loop is provided comprising a delay element that delays the output signal of the multiplexer by one sample clock. A multiplier multiplying the signal by a constant factor is inserted after this delay element. The output signal of the multiplier is added to the input signal delayed by one sample clock. This sum is applied to the second input of the multiplexer which is then activated if the difference between two successive sample values falls short of the predeterminable difference value. So in this case, this feedback loop operates as a low-pass filter, so that the noise is gradually ascertained and in this manner any erroneous initial value is gradually brought to the correct set value of the signal.
Since in response to the factor of the multiplier, the feedback loop, operating as a low-pass filter, presents a gain which is unequal to 1, a second multiplier is inserted before the first input to the multiplexer, which second multiplier multiplies the actual input signal by the the signals available at the two inputs of the multiplexer present the same gain.